Sally can paint a room in 3 hours while it takes Steve 9 hours to paint the same room. How long would it take them to paint the room if worked together?

Step-by-step explanation:

Okay, so- The first thing I did was see how much they got done per hour. Like this:

Sally  ⇒

3 hours = 1 room

1 hour = 1/3 of the room

Steve ⇒

9 hours = 1 room

1 hour = 1/9 of the room

Sally + Steve ⇒

1 hour = 1/3 + 1/9 = 3/9 + 1/9 = 4/9

So, they get 4/9 of a room done in 1 hour.

Number of hours needed ⇒

1 ÷ 4/9 = 2 1/4  

They need 2 1/4 hours to paint the room.

step-by-step explanation: a reaction is first order. if the initial reactant concentration is 0.0200 m, and 25.0 days later the concentration is 6.25 x 10-4 m, then its half-life is:

the largest number of the four options is 1.8 x 10-2

you have it in scientific notation.

1.80 x 10-6   = 0.00000180

1.80 x 10-4   = 0.000180 (bigger, or "less small", than the former one)

so

1.8 x 10-2       = 0.0180. as you see. the "less small", or the biggest one of them all.

as you have negative exponent, it means you are dividing 1.8 by 100 (i.e. moving the dot towards left, two times).

if you had positive exponent for the scientific notation, power of tenths, you would be multiplying times 100 (moving the dot towards right) as below:

1.8 x (10)2 = 180

so. if you have negative exponent in scientific notation, move the dot toward left. if it is positive, toward right as many times the exponent tells you.